1. Field of the Invention
The present invention relates to a process for forming a bolt, for instance, to a process which is appropriate for processing an axially-extendable bolt by upsetting.
2. Description of Related Art
An axially-extendable bolt has been known conventionally. The axially-extendable bolt includes an intermediate leg portion and a threaded leading-end leg portion, and is formed so that the intermediate leg portion has a smaller diameter than that of the threaded leading-end leg portion. As a result, the intermediate leg portion is likely to be extended by an axial tensile force. In general, the axially-extendable bolt has been manufactured in the following manner.
First, a workpiece is prepared by shearing a metallic rod-shaped blank. Then, the resulting workpiece, or a rod-shaped bolt precursor is subjected to drawing by using a forming apparatus. Thus, a leg member having a predetermined diameter is formed, and simultaneously a head member is formed. Moreover, the bolt precursor is subjected to upsetting by using another forming apparatus as illustrated in FIG. 4. Thus, the leg member is processed to have a leading-end leg portion which is subsequently subjected to thread-rolling. The resultant leading-end leg portion of the leg member has a larger diameter than that of the intermediate portion thereof. As illustrated in FIG. 4, the forming apparatus is provided with a die 50, and a knock-out member 60. The die 50 has a through bore 51. A bolt precursor "W" is fitted into the through bore 51 from one of the opposite ends of the through bore 51. The knock-out member 60 is fitted into the through bore 51 from another one of the opposite ends of the through bore 51, and is disposed relatively movably therein. The bolt precursor "W", fitted into the through bore 51, is moved relatively with respect to the knock-out member 60 along with the die 50. Accordingly, the leading end of the bolt precursor "W" is pressed against the leading end of the knock-out member 60. Consequently, the bolt precursor "W" is swollen at the leading end in radial directions. The forming apparatus thus carries out the upsetting operation.
Thereafter, the swollen leading-end leg portion of the bolt precursor "W", formed by the upsetting operation, is subjected to drawing in order to carry out shaping. After the shaping operation, the leading-end leg portion of the precursor "W" is subjected to thread-rolling in order to form screw threads thereon. Finally, the bolt precursor "W" is subjected to hexagonal-head shaping at the head member, round-flange trimming at the head member, a surface treatment, and the like, if necessary. An axially-extendable bolt is thus completed.
When preparing a bolt precursor in the above-described manner, a metallic rod-shaped blank is sheared in the following manner. As illustrated in FIG. 5, a cutter quill 70, and a cutter 80 are first disposed parallelly. The cutter quill 70 has a through bore 71, and the cutter 80 has a through bore 81. The through bores 71 and 81 are communicated with each other linearly. Then, a metallic rod-shaped blank "A" is fitted into the through bores 71 and 81. Finally, the cutter 80 is moved heavily in a shearing direction with respect to the cutter quill 70. However, when the metallic rod-shaped blank "A" is sheared, the cut surface of the resulting bolt precursor "W" is not flat because a sheared surface "d", and a fractured surface "e" arise therein as illustrated in FIG. 6(a). The disadvantages result from the existence of clearances "a", "b" and "c" shown in FIG. 5. The clearances "a" and "b" are formed between the inner peripheral surfaces of the through bores 71 and 81, into which the metallic rod-shaped blank "W" is fitted, and the outer peripheral surface of the metallic rod-shaped blank "W". The clearance "c" is formed between the facing surfaces of the cutter quill 70 and cutter 80. In addition, as illustrated in FIG. 6(b), a running "f" arises on the outer periphery adjacent to the cut surface of the resultant bolt precursor "W". As a result, irregularities arise in portions adjacent to the cut surface of the resultant bolt precursor "W".
When the bolt precursor "W" having the defects is subjected to the upsetting operation as illustrated in FIG. 4, there arises a problem in that a dent "g" is likely to develop in the upset portion because an uneven thickness is likely to occur, and because the upsetting accuracy is prone to be unstable. As illustrated in FIG. 7, when the dent "g" is developed, an oblique drawing takes place eventually in the subsequent drawing operation for shaping the upset portion of the bolt precursor "W". As a result, an axial run-out occurs in the drawn portion, and the perpendicularity deteriorates therein. All in all, there emerges a problem in that these drawbacks finally result in the deterioration in the qualities of a completed bolt.
The applicant of the present invention has proposed a novel process for forming a bolt in Japanese Patent Application No. 7-209,461. The novel bolt forming process is intended for securing a flatness in the leading-end surface of a leg member of a bolt. In the novel bolt forming process, in order to carry out the objective, a bolt precursor is flowed radially outwardly at a portion adjacent to the leading-end surface, and simultaneously the flowed portion is removed by cutting. When the novel bolt forming process is applied to the above-described upsetting operation, it is possible to inhibit the aforementioned uneven thickness from arising, because the flowed portion is removed by cutting. However, it is necessary to carry out post-processing the burrs and flashes, which have been produced by cutting. Consequently, there might arise a drawback in that the man-hour requirement, and the cost of equipment increase.